Urban Computing: Concepts, Methodologies, and Applications

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Urban Computing: Concepts, Methodologies, and Applications 1 YU ZHENG Microsoft Research LICIA CAPRA University College London OURI WOLFSON University of Illinois at Chicago HAI YANG Hong Kong University
Urban Computing: Concepts, Methodologies, and Applications 1 YU ZHENG Microsoft Research LICIA CAPRA University College London OURI WOLFSON University of Illinois at Chicago HAI YANG Hong Kong University of Science and Technology Urbanization s rapid progress has modernized many people s lives, but also engendered big issues, such as traffic congestion, energy consumption, and pollution. Urban computing aims to tackle these issues by using the data that has been generated in cities, e.g., traffic flow, human mobility and geographical data. Urban computing connects urban sensing, data management, data analytics, and service providing into a recurrent process for an unobtrusive and continuous improvement of people s lives, city operation systems, and the environment. Urban computing is an interdisciplinary field where computer sciences meet conventional city-related fields, like transportation, civil engineering, environment, economy, ecology, and sociology, in the context of urban spaces. This article first introduces the concept of urban computing, discussing its general framework and key challenges from the perspective of computer sciences. Secondly, we classify the applications of urban computing into seven categories, consisting of urban planning, transportation, the environment, energy, social, economy, and public safety & security, presenting representative scenarios in each category. Thirdly, we summarize the typical technologies that are needed in urban computing into four folds, which are about urban sensing, urban data management, knowledge fusion across heterogeneous data, and urban data visualization. Finally, we outlook the future of urban computing, suggesting a few research topics that are somehow missing in the community. H.2.8 [Database Management]: Database Applications - data mining, Spatial databases and GIS; J.2 [Physical Sciences and Engineering]: Earth and atmospheric sciences, Mathematics and statistics; J.4 [Social and Behavioral Sciences]: Economics, Sociology; G.1.6 [Optimization]; G.1.2 [Approximation]; E.1 [Data Structures]; E.2 [Data Storage Representations] General Terms: Algorithms, Measurement, Experimentation Additional Key Words and Phrases: urban computing, urban informatics, big Data, human mobility, city dynamics, urban sensing, knowledge fusion, computing with heterogeneous data, trajectories. Authors addresses: Y. Zheng, Microsoft Research, Building 2, No. 5 Danling Street, Haidian District, Beijing , China; L. Capra, Dept. of Computer Science, University College London, Gower Street, London WC1E 6BT, United Kingdom; O. Wolfson, Department of Computer Sciences, University of Illinois at Chicago, Chicago Illinois; H. Yang, Department of the Civil and Environmental Engineering, Hong Kong University of Sciences and Technology, Clear Water Bay, Kowloon, Hong Kong; Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from 2014 ACM /07/0900-ART9 $5.00 DOI / ACM Trans. On Intelligent Systems and Technology, Vol. 6, No. 2, Article 9, Pub. date: July 2014. 1: 2 Y. Zheng, L. Capra, O. Wolfson, H. Yang 1. INTRODUCTION Urbanization s rapid progress has led to many big cities, which have modernized many people s lives but also engendered big challenges, such as air pollution, increased energy consumption and traffic congestion. Tackling these challenges can seem nearly impossible years ago given the complex and dynamic settings of cities. Nowadays, sensing technologies and large-scale computing infrastructures have produced a variety of big data in urban spaces, e.g., human mobility, air quality, traffic patterns, and geographical data. The big data implies rich knowledge about a city and can help tackle these challenges when used correctly. For instance, we can detect the underlying problems in a city s road network through analyzing the city-wide human mobility data. The discovery can help better formulate city planning for the future [Zheng et al. 2011b]. Another example is to exploit the root cause of urban air pollution by studying the correlation between air quality and other data sources, such as traffic flow and points of interests (POIs) [Zheng et al. 2013b]. Motivated by the opportunities of building more intelligent cities, we came up with a vision of urban computing, which aims to unlock the power of knowledge from big and heterogeneous data collected in urban spaces and apply this powerful information to solve major issues our cities face today [Zheng et al. 2012c; 2013a]. In short, we are able to tackle the big challenges in big cities by using big data, as depicted in Figure 1 A). Big Cities The Environment Win Big Data Big Challenges People Win Urban Computing Win Cities OS A) Motivation: Big cities, data and challenges B) Goal of urban computing Figure 1. Motivation and goal of urban computing Though the term of urban computing is not first used in this article [Kindberg et al. 2007; Kostakos and O Neill 2008], it is still a vague concept with many questions open. For example, what are the core research problems of urban computing? What are the challenges of the research theme? What are the key methodologies for urban computing? What are the representative applications in this domain, and how does an urban computing system work? To address these issues, we formally coin in urban computing in this article and introduce its general framework, key research problems, methodologies, and applications. This article will help the community better understand and explore this nascent area, therefore generating quality research results and real systems that can eventually lead to greener and smarter cities. In addition, urban computing is a multi-disciplinary research field, where computer sciences meet conventional city-related areas, such as civil engineering, transportation, economics, energy engineering, environmental sciences, ecology, and sociology. This paper mainly discusses the aforementioned problems from the perspective of computer sciences. The rest of the paper is organized as follows. In Section 2, we introduce the concept of urban computing, presenting a general framework, and the key challenges of each step in the framework. The datasets that are frequently used in urban computing are also briefed. ACM Trans. Intelligent systems and technologies, Vol. 6, No. 3, Article 9, Pub. date: November 2014. Urban Computing: Concepts, Methodologies, and Applications 9: 3 In Section 3, we categorize the applications of urban computing into seven groups, presenting some representative scenarios in each group. In Section 4, we introduce four folds of methodologies that are usually employed in an urban computing scenario. In Section 5, we conclude the article and point out a few future direction of this research theme. 2. FRAMEWORK OF URBAN COMPUTING 2.1 Definition Urban computing is a process of acquisition, integration, and analysis of big and heterogeneous data generated by a diversity of sources in urban spaces, such as sensors, devices, vehicles, buildings, and human, to tackle the major issues that cities face, e.g., air pollution, increased energy consumption and traffic congestion. Urban computing connects unobtrusive and ubiquitous sensing technologies, advanced data management and analytics models, and novel visualization methods, to create win-win-win solutions that improve urban environment, human life quality, and city operation systems, as shown in Figure 1 B). Urban computing also helps us understand the nature of urban phenomena and even predict the future of cities. Urban computing is an interdisciplinary field fusing the computing science with traditional fields, like transportation, civil engineering, economy, ecology, and sociology, in the context of urban spaces. 2.2 General Framework Figure 2 depicts a general framework of urban computing which is comprised of four layers: urban sensing, urban data management, data analytics, and service providing. Using urban anomaly detection as an example [Pan et al. 2013], we briefly introduce the operation of the framework as follows. In the urban sensing step, we constantly probe people s mobility, e.g., routing behavior in a city s road network, using GPS sensors or their mobile phone signals. We also continuously collect the social media people have posted on the Internet. In the data management step, the human mobility and social media data are well organized by some indexing structure that simultaneously incorporates spatio-temporal information and texts, for supporting efficient data analytics. In the data analytics step, once an anomaly occurs, we are able to identify the locations where people s mobility significantly differs from its origin patterns. In the meantime, we can describe the anomaly by mining representative terms from the social media that is related to the locations and time span. In the service providing step, the locations and description of the anomaly will be sent to the drivers nearby so that they can choose a bypass. In addition, the information will be delivered to the transportation authority for dispersing traffic and diagnosing the anomaly. The system continues the loop for an instant and unobtrusive detection of urban anomalies, helping improve people s driving experiences and reduce traffic congestion. Compared with other systems, e.g., web search engines which are based on a single (modal)-data-single-task framework (i.e., information retrieval from web pages), urban computing holds a multi (modal)-data-multi-task framework. The tasks of urban computing include improving urban planning, easing traffic congestion, saving energy consumption, and reducing air pollution, etc. Additionally, we usually need to harness a diversity of data sources in a single task. For instance, the aforementioned anomaly detection uses human mobility data, road networks, and social media. Different tasks can be fulfilled by combining different data sources with different data acquisition, management and analytics techniques from different layers of the framework. ACM Trans. Intelligent Systems and Technology, Vol. 6, No. 3, Article 9, Pub. date: November 2013. 1: 4 Y. Zheng, L. Capra, O. Wolfson, H. Yang Service Providing Improve urban planning, Ease Traffic Congestion, Save Energy, Reduce Air Pollution,... Urban Data Analytics Data Mining, Machine Learning, Visualization Urban Data Management Spatio-Temporal Index, Stream, Trajectory, and Graph Data Management.. Human mobility Traffic Air Quality Meteorolo gy Social Media Energy Road Networks POIs Urban Sensing & Data Acquisition Participatory Sensing, Crowd Sensing, Mobile Sensing Figure 2. General framework of urban computing 2.3 Key Challenges The goals and framework of urban computing result in three folds of main challenges: Urban sensing and data acquisition: The first is data acquisition techniques that can unobtrusively and continually collect data in a citywide scale. This is a non-trivial problem given the three italic terms. Monitoring the traffic flow on a road segment is easy; but continually probing the citywide traffic is challenging as we do not have sensors on every road segment. Building new sensing infrastructures could achieve the goal but would aggravate the burden of cities in turn. How to leverage what we already have in urban spaces intelligently is a way yet to explore. Human as a sensor is a new concept that may help tackle this challenge. For instance, when users post social media on a social networking site, they are actually helping us understand the events happening around them. When many people drive on a road network, their GPS traces may reflect the traffic patterns and anomalies. However, as a coin has two sides, despite the flexibility and intelligence of human sensors, human as a sensor also brings three challenges (we will discuss more about this part in Section 4.1): Energy consumption and privacy: It is a non-trivial problem for participatory sensing applications, where users proactively contribute their data (usually using a smart phone), to save the energy of a smart phone and protect the privacy of a user during the sensing process. There is a trade-off among energy, privacy and the utility of shared data [Xue et al. 2013]. Loose-controlled and non-uniform distributed sensors: We can put traditional sensors anywhere we like and configure these sensors to send sensing readings at a certain frequency. However, we cannot control people who would send information anytime they like or do not share data sometimes. In some places, we may not even have people at some moments, i.e., could not have sensor data, inevitably resulting in data missing and sparsity problems. On the other hand, ACM Trans. Intelligent systems and technologies, Vol. 6, No. 3, Article 9, Pub. date: November 2014. Urban Computing: Concepts, Methodologies, and Applications 9: 5 the user-generated content in some location (with many people) maybe over sufficient or even redundant, adding unnecessary workload for sensing, communication, and storage. Additionally, what we can obtain is always a sample of data from partial users, as not everyone shares data. The distribution of the sample data may be skewed from the distribution of the entire dataset, depending on the movement of people. Unstructured, implicit, and noise data: The data generated by traditional sensors is well structured, explicit, clean and easy to understand. However, the data contributed by users is usually in a free format, such as texts and images, or cannot explicitly lead us to the final goal as if using traditional sensors. Sometimes, the information from human sensors is also quite noisy. Using the application presented in [Zhang et al. 2013] as an example, we illustrate the two challenges. In this example, Zhang et al. aim to use GPS-equipped taxi drivers as sensors to detect the queuing time in a gas station (when they are refueling taxis) and further infer the number of people who are also refueling their vehicles there. The goal is to estimate the gas consumption of a station and finally the citywide gas consumption in a given time span. In this application, what we obtain is the GPS trajectories of a taxi driver, which does not tell us the result explicitly. In addition, we cannot guarantee having a taxi driver in each gas station anytime, which results in a data missing problem. In the meantime, the presences of taxis in a station may be quite different from that of other vehicles (i.e., the skewed distribution); e.g., observing more taxis in a gas station does not denote more other vehicles. Furthermore, taxi drivers may park taxis somewhere close to a gas station just for having a rest or waiting for a traffic light. These observations from the GPS trajectory data are noisy. In short, we usually need to learn what we really need, from partial, skewed, noisy, and implicit data generated by human sensors. Computing with heterogeneous data: Learn mutually reinforced knowledge from heterogeneous data: Solving urban challenges needs to oversee a broad range of factors, e.g., to explore air pollutions needs simultaneously study the traffic flow, meteorology, and land uses. However, existing data mining and machine learning techniques usually handle one kind of data, e.g., computer vision is dealing with images, and natural language processing is based on texts. According to the studies [Zheng et al. 2013b; Yuan et al. 2012], equally treating features extracted from different data sources (e.g., simply putting these features into a feature vector and throw them into a classification model) does not achieve the best performance. In addition, using multiple data sources in an application leads to a high-dimension space, which usually aggravates data sparsity problem. If not handled correctly, more data sources would even compromise the performance of a model. This is calling for advanced data analytics models that can learn mutually reinforced knowledge among multiple heterogeneous data generated from different sources, including sensors, people, vehicles, and buildings. See Section 4.1 for more details. Both effective and efficient learning ability: Many urban computing scenarios, e.g., detecting traffic anomalies and monitoring air quality, need instant answers. Besides just increasing the number of machines to speed up the computation, we need to aggregate data management, mining and machine learning algorithms into a computing framework to provide a both effective and efficient knowledge discovery ability. In addition, traditional data management techniques are ACM Trans. Intelligent Systems and Technology, Vol. 6, No. 3, Article 9, Pub. date: November 2013. 1: 6 Y. Zheng, L. Capra, O. Wolfson, H. Yang usually designed for a single modal data source. Advanced management methodology that can well organize multi-modal data (such as streaming, geospatial, and textual data) is still missing. So, computing with multiple heterogeneous data is a fusion of data and also a fusion of algorithms. See Section 4.3 for more discussions. Visualization: Massive data brings a tremendous amount of information that needs a better presentation. A good visualization of original data could inspire new idea to solve a problem, while the visualization of computing results can reveal knowledge intuitively so as to help a decision making. The visualization of data may also suggest the correlation or causality between different factors. The multimode data in urban computing scenarios leads to high dimensions of views, such as spatial, temporal, and social, for a visualization. How to interrelate different kinds of data in different views and detect patterns and trends is challenging. In addition, when facing multiple types and huge volume of data, how exploratory visualization [Andrienko et al. 2003] can provide an interactive way for people to generating new hypothesis becomes even more difficult. This is calling for an integration of instant data mining techniques into a visualization framework, which is still missing in urban computing. Hybrid systems blending the physical and virtual worlds: Unlike a search engine or a digital game where the data was generated and consumed in the digital world, urban computing usually integrates the data from both worlds, e.g., combining traffic with social media. Alternatively, the data (e.g., GPS trajectories of vehicles) was generated in the physical world, and then sent back to the digital world, such as a Cloud system. After processed with other data sources in the Cloud, the knowledge learned from the data will be used to serve users from the physical world via mobile clients, e.g., driving direction suggestion, taxi ridesharing, and air quality monitoring. The design of such a system is much more challenging than conventional systems that only reside in one world, as the system needs to communicate with many devices and users simultaneously, send and receive data of different formats and at different frequencies. 2.4 Urban Data In this section, we introduce the frequently used data sources in urban computing and briefly mention the issues we usually face when using these data sources Geographical Data Road network data may be the most frequently used geographical data in urban computing scenarios, e.g., traffic monitoring and prediction [Pan and Zheng et al. 2013], urban planning [Zheng et al. 2011b], routing [Yuan and Zheng et al. 2010a; 2011b; 2013b], and energy cons
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